Conventionally, a cold cathode fluorescent lamp (CCFL: Cold Cathode Fluorescent Lamp) has been used as a light source for a backlight in a liquid crystal display device or the like. However, in the case when the cold cathode fluorescent lamp is used as a light source for a backlight, there exist problems such that color reproducibility is low, and further an environmental burden is large because mercury is used.
On the other hand, in recent years, a blue light emitting diode chip and a light emitting diode chip that is capable of emitting ultraviolet light have been developed commercialized, and LED illumination has been actively developed. For the LED illumination, there exist three methods, which are a method in which a B-light emitting diode chip and a yellow light emitting phosphor are combined, a method in which lights of an R-light emitting diode chip, a G-light emitting diode chip, and a B-light emitting diode chip are mixed, and a method in which an UV-light emitting diode chip emitting ultraviolet light and RGB phosphors are combined. The former two methods each have problems with regard to points of color reproducibility, and color controllability color mixture, and thus the method of an UV-light emitting diode chip+RGB phosphors has been increasingly expected.
A light source using a light emitting diode chip has long life and high reliability, and an exchange work and the like become easier to be performed, so that it has been expected to be used as a component part of various display devices. A color tone of light emitted from the light source depends not only on a light emission wavelength of the light emitting diode chip, and lights in a visible light spectrum from blue to red depending on intended uses can be obtained according to types of phosphors to be contained in a phosphor layer to be formed on a front surface or the like of the light emitting diode chip. Particularly, a white-light-emitting-type light source has been expected to be used as a light source for a backlight of a liquid crystal display device in a portable communication device, a liquid crystal display device in a PC, and the like.
The backlight is mainly classified into a direct-type backlight and a sidelight-type backlight, depending on a position where the light source is disposed, and these backlights are used properly according to their usage. Between these backlights, the sidelight-type backlight has a light source disposed on a side surface to be an incident surface of a light guide plate. As the light source of the sidelight-type backlight, for example, a plurality of light emitting diodes to be disposed on an incident side surface of a light guide plate at predetermined intervals have been known (see, for example, JP-A 2000-258749 (KOKAI)).
However, the light source using the light emitting diode chips as described above, because a size of each of the light emitting diode chips being an excitation light source is 1 mm2 or less, almost becomes a point light source even though a phosphor layer is formed on the light emitting diode chips. Thus, in the case when the light source using the light emitting diode chips as described above are used as a light source for illumination, a backlight and the like, it is necessary to provide a lens-shaped transparent resin layer, a diffusion sheet or the like to diffuse light to be emitted.
Even in the case when such a lens-shaped transparent resin layer, diffusion sheet or the like is provided, it is not always possible to sufficiently reduce a difference in luminance between a portion having the light emitting diode chip provided thereon and a portion sandwiched between the portions having the light emitting diode chips each provided thereon, and a process of providing such a lens-shaped transparent resin layer, diffusion sheet, or the like is needed, resulting that manufacturability is also reduced. Further, in the case when the light source using the light emitting diode chips as described above are used as a light source for illumination, a backlight and the like in place of a conventional cold cathode fluorescent lamp, a shape thereof differs from that of the cold cathode fluorescent lamp, resulting that a new optical design is required with regard to illumination and a backlight.